1. Field of the Invention
The present invention relates to an endless belt-shaped metal substrate that can be used as a substrate of a fixing member having an endless belt shape to be used for heat fixing of an electrophotographic image forming apparatus and the like. The present invention also relates to a fixing member and a heat-fixing device, which use the endless belt-shaped metal substrate.
2. Description of the Related Art
Currently, some heat-fixing devices included in electrophotographic image forming apparatus adopt a belt heating system capable of suppressing power consumption.
FIG. 2 is a sectional view illustrating a schematic configuration of a typical belt heating type heat-fixing device. The heat-fixing device includes a fixing belt 11 serving as a fixing member, a pressure roller 20 serving as a pressure member disposed so as to be opposed to the fixing member, and a ceramic heater 12 serving as a heating unit disposed in contact with an inner peripheral surface of the fixing belt 11. The fixing belt 11 and the pressure roller 20 form a fixing nip part N, and a recording material 30 bearing an unfixed toner image T is guided into the fixing nip part N so that toner forming the unfixed toner image T is melted to fix a toner image onto the recording material 30.
FIG. 3 is a sectional view of the fixing member (hereinafter sometimes referred to as “fixing belt”) 11 having an endless belt shape. The fixing belt 11 is formed of substantially three layers of a substrate 101, an elastic layer 102, and a surface layer 103 such as a releasing layer in the stated order from the ceramic heater 12 side. As the substrate 101, an endless belt-shaped thin metal substrate (hereinafter sometimes referred to as “metal belt”) having high heat conductivity is used.
FIG. 1 is an explanatory view of the fixing belt 11 that is subjected to bending strain in the belt heating type heat-fixing device. In the belt heating system, when the fixing belt 11 passes through the fixing nip part N, the fixing belt 11 is subjected to bending in a circumferential direction on an inlet side and an outlet side. The fixing belt 11 conveyed by a driving roller is repeatedly inserted into the fixing nip part N and thus is repeatedly subjected to the bending.
When the fixing belt 11 is repeatedly subjected to the bending, a fatigue failure may be caused in the metallic substrate of the fixing belt 11. Therefore, in order to enhance the durability of the fixing belt 11, it is necessary to further improve bending resistance also in the metal belt.
In this case, as a material for the metal belt, a relatively inexpensive austenitic stainless-steel sheet having satisfactory processability is used (Japanese Patent Application Laid-Open No. 2005-241891).
A metastable austenitic stainless-steel sheet such as SUS304 can be thinned by plastic forming relatively easily. Further, in such a stainless-steel sheet, at least a part of an austenite phase is transformed into a martensite phase due to plastic forming (hereinafter sometimes referred to as “strain induced transformation”). The martensite phase formed by the strain induced transformation (hereinafter sometimes referred to as “strain induced martensite phase”) has a high hardness as compared to the austenite phase, and hence can be formed into a metal substrate having a relatively high surface hardness.
In a metal belt obtained by subjecting a stainless-steel sheet having a high transformation ratio of an austenite phase into a martensite phase after plastic forming (hereinafter referred to as “martensite transformation ratio”) to plastic forming, an increase in tensile strength and suppression of a fatigue failure caused by repeated bending are expected due to the formation of the strain induced martensite phase, as described above.
However, according to the investigations conducted by the present inventors, although the metal belt having a martensite ratio increased by strain induced transformation exhibits high tensile strength, there is a high probability that a fatigue failure such as a crack may occur when the metal belt is repeatedly bent. The present inventors have recognized that, in particular, in the case where the stainless-steel sheet is processed into a thin metal belt having a thickness of from 10 to 100 μm, the number of repetitions before the fatigue failure is caused by the repeated bending (product lifetime) varies significantly, resulting in poor reliability.
In view of the foregoing, the present invention is directed to providing a metal substrate for a fixing member in which a fatigue failure is not easily caused by repeated bending.
Further, the present invention is also directed to providing a fixing member and a heat-fixing device, which contribute to the stable formation of a high-quality electrophotographic image.